Description: This application aims to advance understanding of selected fundamental properties of long-axoned (propriospinal) spinal (SC) interneurons (INs) in several mammalian species (adult mouse, transgenic mouse, neonatal rat, adult cat) and preparations (intact and freely moving, reduced in vivo, isolated in vitro SC and SC slices). The focus is on narrowing the gap between current understanding of mammalian spinal INs and identified INs in invertebrate preparations. There is an emphasis on INs which may participate in spinal pattern-generating activity for the elaboration of locomotion. Such INs include two potentially overlapping groups which are the focus of this proposal: cholinergic INs and 5-HT-activated ones. It is hoped to advance understanding of these cell's spinal location, responses to descending and sensory input, selected pharmacological properties, non-neuromodulated and neuromodulated passive, transitional, and active (repetitive-firing) properties, and their effects on their output spinal motoneurons (MNs).